House Majority Leader Eric Cantor listens to House Speaker John Boehner speak after attending the weekly House Republican conference at the U.S. Capitol March 25, 2014 in Washington, DC.

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The latest battleground in the civil war between the Republican establishment and the conservative grass roots is a tony Florida island with 13 miles of coastline and six golf courses.

Amelia Island is playing host this weekend to the annual retreat held by the Republican Main Street Partnership, an arm of a super PAC that helps Republican incumbents fend off tea-party challengers. Some conservatives are agitating about the scheduled attendance of two top-ranking House Republicans, House Majority Leader Eric Cantor and Majority Whip Kevin McCarthy.

Brent Bozell, who heads the ForAmerica conservative nonprofit, warned Mr. Cantor in a letter that his participation would “permanently destroy any credibility you have left with conservatives.” ForAmerica is also using social media to urge supporters to “call Eric Cantor and tell him to stop attacking conservatives.” Mr. Cantor is next in line to be the House Speaker if the GOP retains control of the chamber in the 2014 elections.

A spokesman for Mr. Cantor, Douglas Heye, said his participation in the policy-oriented retreat is in line with conservative goals. “This is classic judging a speaker by the audience,” Mr. Heye said. “We need to present conservative solutions to all audiences, and grow both our grass roots and our majority so we can stop President Obama and Nancy Pelosi’s liberal agenda.”

Why would conservatives like Mr. Bozell see Republican Main Street Partnership–which counts about 51 House Republicans as members–as an enemy? First of all, they don’t like that the super PAC takes money from labor unions, at least $350,000 this year, according to the Center for Responsive Politics. Conservative blogger Erick Erickson wrote Wednesday that the “group is funded by the vast left-wing conspiracy.”

Tea-party leaders also don’t like the group’s interference in races where the Republican incumbent has been deemed insufficiently conservative. This cycle, the super PAC has spent $200,000 to defend Republican Rep. Mike Simpson (R., Idaho) against a tea-party challenger, and $44,000 to attack Republican Matt Lynch, who is trying to unseat Rep. Dave Joyce (R., Ohio) in the Republican primary.

Mr. Joyce is the successor to former Republican Rep. Steve LaTourette, president of the super PAC and a board member of the affiliated group hosting the retreat. Mr. LaTourette called the objections “beyond ridiculous.”

He added, “Republican leadership attending the conference doesn’t mean we’re going feed them Kool-Aid and make them squishy moderates. They just want to be inclusive and listen to what we have to say.”

The congressmen seem unlikely to be dissuaded from going. But Mr. Bozell’s protest may extract some gains: visitors to the ForAmerica website viewing a copy of his letter to Mr. Cantor are directed to make a contribution.

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